CHAPTER VII. 



THE EVIDEXCE FROM YARIATIOJT. 



Causes of variation — Changed conditions of life induce varia- 

 bility — No particular kind of change is necessarj* — Variabil- 

 ity is almost exclusively confined to organisms produced 

 fi'om fertilized ova — Bud variation very raie — History of 

 the Italian orange — The frequency of variation in organ- 

 isms produced from sexual union, as compared with its infre- 

 quency in those produced asexually, receives a direct expla- 

 nation by our theor}-^ of hereditj'- — Bud variation more 

 frequent in cultivated than in wild plants — Our theory 

 would lead us to expect this — Clianged conditions do not 

 act directlj', but they cause subsequent geneiations to vary 

 — Tendency to var}' is hereditary — These facts perfectly ex- 

 plicable by our theory — Specific characters more variable 

 than generic — Species of large genera more variable than 

 those of small genera — A part developed in an unusual way 

 highly variable — Law of equable variation — Secondary sex- 

 ual characters variable — Natural selection cannot act to 

 produce permanent modification unless many individuals 

 vary together — Our theory is the only explanation of the 

 simultaneous variation of many individuals — This theory 

 also simplifies the evolution of complex structures — Salta- 

 tory evolution — This is explained by our theory of heredity 

 — Correlated variation of homologous parts — Parts confined 

 to males more variable than parts confined to females — 

 Males more variable than females — Summary of last two 

 chapters. 



Tlie Causes of Variation. 



Certaiisj' authors have lield tliat variabilitv is a neces- 

 sary accompaniment of reproduction; that it is deter- 

 mined by something within rather than without the or- 



